Second-degree atrioventricular block
|Second-degree atrioventricular block|
|Classification and external resources|
The presence of second-degree AV block is diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction.
There are two non-distinct types of second-degree AV , called Type 1 and Type 2. In both types, a P wave is blocked from initiating a QRS complex; but, in Type 1, there are increasing delays in each cycle before the omission, whereas, in Type 2, there is no such pattern.
Type 1 second-degree heart block is considered a more benign entity than type 2 second-degree heart block.
Type 1 (Mobitz I/Wenckebach)
Mobitz I heart block is characterized by progressive prolongation of the PR interval on the electrocardiogram (ECG) on consecutive beats followed by a blocked P wave (i.e., a 'dropped' QRS complex). After the dropped QRS complex, the PR interval resets and the cycle repeats.
One of the baseline assumptions when determining if an individual has Mobitz I heart block is that the atrial rhythm has to be regular. If the atrial rhythm is not regular, there could be alternative explanations as to why certain P waves do not conduct to the ventricles.
Type 2 (Mobitz II/Hay)
Type 2 Second-degree AV block, also known as "Mobitz II," is almost always a disease of the distal conduction system (His-Purkinje System).
Mobitz II heart block is characterized on a surface ECG by intermittently nonconducted P waves not preceded by PR prolongation and not followed by PR shortening. The medical significance of this type of AV block is that it may progress rapidly to complete heart block, in which no escape rhythm may emerge. In this case, the person may experience a Stokes-Adams attack, cardiac arrest, or sudden cardiac death. The definitive treatment for this form of AV Block is an implanted pacemaker.
The impairment is usually below the AV node. Although the terms infranodal block or infrahisian block are often applied to this disorder, they refer to the anatomic location of the block, whereas Mobitz II refers to an electrocardiographic pattern.
Most people with Wenckebach (Type I Mobitz) do not show symptoms. However, those that do usually display one or more of the following:
- Syncope (Fainting)
- Electrical conduction system of the heart
- Electrocardiogram (ECG or EKG)
- SA node
- AV node
- Atrioventricular block
- First-degree AV block
- Third-degree AV block
- "Lesson VI - ECG Conduction Abnormalities". Retrieved 2009-01-07.
- synd/2824 at Who Named It?
- W. Mobitz. Über die unvollständige Störung der Erregungsüberleitung zwischen Vorhof und Kammer des menschlichen Herzens. Zeitschrift für die Gesamte Experimentelle Medizin, Berlin 1924, 41: 180–237.
- K. F. Wenckebach. De Analyse van den onregelmatigen Pols. III. Over eenige Vormen van Allorythmie en Bradykardie. Nederlandsch Tijdschrift voor Geneeskunde, Amsterdam, 1898, 2: 1132.
- Silverman ME, Upshaw CB, Lange HW (August 2004). "Woldemar Mobitz and His 1924 classification of second-degree atrioventricular block". Circulation 110 (9): 1162–7. doi:10.1161/01.CIR.0000140669.35049.34. PMID 15339865.
- J. Hay. Bradycardia and cardiac arrhythmia produced by depression of certain of the functions of the heart. The Lancet 1906, 1: 139–143.
- Lilly, L. S., Pathophysiology of Heart Disease. Baltimore: Lippincott Williams & Wilkins; 2007
- Wogan JM, Lowenstein SR, Gordon GS (1993). "Second-degree atrioventricular block: Mobitz type II". J Emerg Med 11 (1): 47–54. doi:10.1016/0736-4679(93)90009-V. PMID 8445186.